Downhole well instrumentation is often used to monitor the direction in which a well is being drilled. Typically, instruments such as magnetometers and inclinometers are lowered down into a borehole being drilled. The instruments monitor the orientation of the assembly lowered downhole with respect to the directions of the magnetic and gravitational forces of the earth, thereby providing useful information as to the direction in which the borehole is being drilled. To interpret the information produced by the downhole instrumentation, however, it is necessary that the orientation of the instrumentation with respect to the borehole be known.
One way of establishing a fixed positional relationship between the instrumentation and the borehole is to align the instrumentation with the casing of the borehole using, for example, a mule shoe. As is well known, a mule shoe is a structure, terminating the instrumentation assembly, which includes inclined guide surfaces and an alignment slot. The mule shoe is fixed in angular position with respect to the instrumentation assembly. A projection (such as a pin or the like) is disposed on the inner surface of the borehole casing. When the instrumentation assembly is lowered into the borehole, the projecting pin engages the guide surfaces of the mule shoe, causing the entire instrumentation assembly to rotate to a position wherein the projecting pin engages the alignment slot of the mule shoe. The instrumentation assembly is thereby aligned to a predetermined known angular orientation with respect to the borehole.
Instrumentation assemblies typically by necessity comprise a plurality of respective segments or units. For instance, it is usually desirable to encase part of the instrumentation assembly in a pressure-proof "barrel" (e.g. cylinder) to protect it from the great pressures which are often encountered downhole, while maintaining other parts (such as the mule shoe) in direct contact with the borehole casing. The instrumentation cannot usually be manufactured as an integral unit since repair and replacement of the instrumentation would thereby become exceedingly difficult. Connections or other mechanical coupling means are therefore required to mechanically couple the instrumentation assembly to the mule shoe and to the pressure barrel.
Any means used to couple the instrumentation assembly to the mule shoe (or other borehole alignment means) may not introduce an unknown angular displacement into the angular relationship between the assembly and the mule shoe. If such an angular displacement is introduced, the assembly will no longer be precisely aligned with the projection for the borehole casing, and error in the interpretation of the sensed orientation of the borehole with respect to the earth's fields will result. Likewise, means used to connect any section of the instrumentation assembly to another may not introduce unknown angular displacements into the relative orientation between the connected segments.
Although it is typically not possible to achieve zero angular displacement between two connected sections, it is only necessary for accurate measurement that the angular displacement between the sections be known. The angular displacement between the sections can be accurately measured on the surface, and the measurements performed by the downhole instrumentation can be adjusted to compensate for the angular displacement to yield accurate results. However, it has been found that due to the wear caused by simply mating and unmating conventional connections used to connect segments together, the angular displacement which they introduce may change appreciably over time. The rate of wear of a connection is increased drastically if the connection is not mated securely (thereby permitting one part of the connection to vibrate with respect to the other part as the instrumentation assembly is lowered downhole). Wear of connection mating surfaces on the order of less than one thousandth of an inch is enough to appreciably change the angular displacement which the connection introduces into the instrumentation assembly, thereby introducing substantial inaccuracy into the orientation measurement performed by the assembly.
It is typically difficult or impossible for measurement of the angular displacement introduced by the connection to be performed in the field to the accuracy required. Hence, the practice in the industry is to periodically send instrumentation assemblies, including connections, to a test lab where accurate calibration can be performed. In order to reduce equipment down time, maintenance costs, etc., it is desirable that recalibration need not be required often. Therefore, an arrangement for connecting two sections of a downhole apparatus which introduces an angular displacement which does not change appreciably with wear would highly be desirable.